feat(compact-basic): baseline compaction backend (squashed from compact-basic)

Collapses the per-round review churn of the prior compact-basic branch into a
single clean baseline on top of compact-interface, so the upcoming retention
refactor lands as fresh, well-scoped commits rather than stacking on a history
of fixes that are being superseded.

tsconfig.json

@@ -38,6 +38,7 @@
     { "path": "./packages/bash/bash-local" },
     { "path": "./packages/bash/tool-bash" },
     { "path": "./packages/compact/compact" },
+    { "path": "./packages/compact/compact-basic" },
     { "path": "./packages/support/invariants" },
     { "path": "./packages/ui/acp" },
     { "path": "./packages/ui/acp-agent" },